1. Field of the Invention
The invention relates generally to a current-perpendicular-to-the-plane (CPP) magnetoresistive (MR) sensor that operates with the sense current directed perpendicularly to the planes of the layers making up the sensor stack, and more particularly to a CPP-MR sensor with side shields.
2. Background of the Invention
One type of conventional magnetoresistive (MR) sensor used as the read head in magnetic recording disk drives is a “spin-valve” sensor based on the giant magnetoresistance (GMR) effect. A GMR spin-valve sensor has a stack of layers that includes two ferromagnetic layers separated by a nonmagnetic electrically conductive spacer layer, which is typically copper (Cu). One ferromagnetic layer adjacent the spacer layer has its magnetization direction fixed, such as by being pinned by exchange coupling with an adjacent antiferromagnetic layer, and is referred to as the reference layer. The other ferromagnetic layer adjacent the spacer layer has its magnetization direction free to rotate in the presence of an external magnetic field and is referred to as the free layer. With a sense current applied to the sensor, the rotation of the free-layer magnetization relative to the reference-layer magnetization due to the presence of an external magnetic field is detectable as a change in electrical resistance. If the sense current is directed perpendicularly through the planes of the layers in the sensor stack, the sensor is referred to as a current-perpendicular-to-the-plane (CPP) sensor.
In addition to CPP-GMR read heads, another type of CPP-MR sensor is a magnetic tunnel junction sensor, also called a tunneling MR or TMR sensor, in which the nonmagnetic spacer layer is a very thin nonmagnetic tunnel barrier layer. In a CPP-TMR sensor the tunneling current perpendicularly through the layers depends on the relative orientation of the magnetizations in the free and reference ferromagnetic layers. In a CPP-GMR read head the nonmagnetic spacer layer is formed of an electrically conductive material, typically a metal such as Cu. In a CPP-TMR read head the nonmagnetic spacer layer is formed of an electrically insulating material, such as TiO2, MgO, or Al2O3.
The sensor stack in a CPP-MR read head has an edge that faces the disk with a width referred to as the track width (TW). The sensor stack has a back edge recessed from the edge that faces the disk, with the dimension from the disk-facing edge to the back edge referred to as the stripe height (SH). The sensor stack is generally surrounded at the TW edges and back edge by insulating material. A layer of hard magnetic material is used to bias or stabilize the magnetization of the free layer and is deposited on both sides of the sensor onto insulating material on each side of the TW edges. As the data density increases in magnetic recording disk drives, there is a requirement for a decrease in the read head dimensions, more particularly the TW. However, the effective or “magnetic” TW does not decrease linearly with a decrease in the physical TW because of side reading of data bits from adjacent tracks. To overcome this problem, side shields of soft magnetically permeable material located on the sides of the sensor have been proposed to absorb magnetic flux from data bits in adjacent tracks. The side shields require that the soft magnetic material be located on the sides of the free layer at the TW edges, which means that the hard magnetic biasing material must be removed. This requires the use of an alternative technique to maintain magnetic stabilization of the free layer.
The sensor stack in a CPP-MR read head is located between two along-the-track top and bottom shields of soft magnetically permeable material that shield the read head from recorded data bits along the track that are neighboring the data bit being read. As the read head dimensions decrease, there is an increasing need to shield the sensor from flux from the neighboring bits in the along-the-track direction as well as from bits in adjacent tracks to improve the spatial resolution of the sensor. During a read operation, the top and bottom shields ensure that the sensor reads only the information from the bit stored directly beneath it on a specific track of the disk by absorbing any stray magnetic fields emanating from adjacent bits and adjacent tracks. In each shield there can be a large number of magnetic domains separated from each other by domain walls. The application of an external magnetic field, such as from bits in adjacent tracks or adjacent bits in the track being read, can cause these magnetic domain walls to move. The overall result is undesirable noise during the read operation.
What is needed is a CPP-MR sensor with side shields that also maintain magnetic stabilization of the free layer and with along-the-track top and bottom shields that reduce noise during a read operation.